Midlevel ventilation's constraint on tropical cyclone intensity

Abstract

Midlevel ventilation, or the flux of low-entropy air into the inner core of a tropical cyclone (TC), is a hypothesized mechanism by which environmental vertical wind shear can constrain a TC's intensity. An idealized framework is developed to assess how ventilation affects TC intensity via two pathways: downdrafts outside the eyewall and eddy fluxes directly into the eyewall. Three key aspects are found: ventilation has a detrimental effect on TC intensity by decreasing the maximum steady state intensity, imposing a minimum intensity below which a TC will unconditionally decay, and providing an upper ventilation bound beyond which no steady TC can exist. Based on the idealized framework, a ventilation index is derived that is equal to the environmental vertical wind shear times the midlevel entropy deficit divided by the potential intensity. The ventilation index has a strong influence on the present-day climatology of tropical cyclogenesis and the distribution of TC intensification. Additionally, changes in the ventilation index are also examined in general circulation models (GCMs) between the late 20th century and the late 22nd century. Individual GCMs indicate potential regional shifts in preferred locations of tropical cyclogenesis and changes in TC intensity statistics due to shifts in the seasonal ventilation index, but a statistically significant projection cannot be given. The GCMs do show a robust increase in the midlevel entropy deficit and potential intensity nearly everywhere in the tropics. Lastly, an axisymmetric model with parameterized ventilation is used to examine the sensitivity of TC intensity to the strength and location of the ventilation and to examine the findings of the idealized framework. Increasing the strength of the ventilation and placing the ventilation at lower to middle levels results in a greater decrease in the quasi-steady intensity, whereas upper-level ventilation has little effect on the intensity. For strong ventilation, an oscillatory intensity regime materializes and is tied to transient convective bursts and strong downdrafts into the boundary layer. The sensitivity of TC intensity to ventilation can be viewed in the context of a modified thermal wind relation or the fractional Carnot efficiency of the inner-core.